FE1: Jake and Beck, for anyone who doesn't know, ballista in FE1 are two ranged locked like regular archers, and are stuck with 4 movement and worse terrain penalties than armor knights.
FE2: Maybe Forsyth, Atlas, or Valbar. Forsyth is discount Lukas, which wouldn't be so bad, but Lukas' big niche is being helpful early on, so a late joining Lukas is redundant. Atlas is a villager who joins relatively late and can be made into something like a Dread Fighter, but he requires much more investment than the three other Dread Fighters gotten before. And Valbar is put in a unfortunate area, where you already had Saber as a good tank, and while he can tank decently, you're immediately greeted to harsh terrain in desert maps and swamp maps, plus countless magic users that result in him being a sitting duck.
FE3 (Book 2 in particular): Roshea, he's the second to last paladin in a game flooded with cavaliers and paladins, he lacks the ability to use silver swords at base, he's immediately forced to dismount after recruited further nerfing his stats, and speaking of his stats, his stats are worse than Midia, Abel, and they're even just on par if not worse than Sirius who joined 15 maps ago.
FE4: Arden/Hannibal, there isn't much discussion here, low movement armors in a game that favors movement.
FE5: Miranda, the only unpromoted mage who doesn't promote into a sage hurts her a lot. She doesn't have the advantage of high promo bonuses or staves, and when she has to dismount indoors she has less movement than her unpromoted class. She also joins halfway through the game and recruiting her means not recruiting Sleuf, Misha, or Amalda.
FE6: Wendy/Sophia, The previous replies have already explained these two units in depth, I assume.
FE7: Karla, Joins too late, is a swordmaster in a game that's bias against swords, and requires training up another underwhelming unit.
FE8: Amelia/Ewan: Both have the same issue of joining late with underwhelming performance, again, all the other replies in this thread have already went into detail about one or the other.
FE9: I'm not too familiar with FE9, but I'm inclined to say maybe Rolf or Sothe.
FE10: haven't played
FE11: Macellan. In chapter 8 you recruit Roger, who is supposed to be a replacement of Draug, when Draug isn't even that good to begin with. Then you get Dolph as a replacement of Roger, then Macellan as a replacement of Dolph. You can probably already visualize this, but Macellan is just worse than all the previous knights before in both bases and growths, and even when evaluating him in a vacuum, he has low weapon ranks for the time he joins in the game, and being stuck in class B, he doesn't have access to much classes to take advantage of the weapon rank he does have.
FE12: Bantu. One thing that's true about all the previous units is that even if the payout isn't a lot, you can theoretically train and make them workable. That isn't the case for Bantu in FE12. Bantu on join have extremely low bases, notably his hp that would get him one rounded by chapter 1 enemies, even with the dragonstone bonuses he gets one rounded in his join map by every enemy aside from Roger. His extremely low growths, so no matter how much you train him, he will most likely remain the same. No access to promotion or reclassing. And because you can't buy more firestones on H3 and up, if you do go through with trying to train him, if his firestone breaks, he quite literally cannot fight anymore. Bantu is the biggest nerf I've ever witnessed from a unit in a fire emblem remake.
FE13-14: haven't played
FE15: See FE2
FE16: Have not gotten far enough to form an opinion.